1. Field of the Invention
This invention relates to a vehicle suspension system, and more particularly to an active suspension system comprising a fluid pressure cylinder device provided between each wheel and a vehicle body.
2. Description of the Prior Art
As disclosed, for instance, in U.S. Pat. No. 4,830,397, there has been proposed a so-called active suspension which comprises fluid pressure cylinder devices provided between the respective wheels and the vehicle body and in which supply and discharge of hydraulic fluid to and from the fluid pressure cylinder devices (will be simply referred to as "fluid supply to the fluid pressure cylinder devices", hereinbelow) is controlled according to the running condition of the vehicle to afford better driving comfort and better running stability.
In such an active suspension system, a control system controls fluid supply to the fluid pressure cylinder devices according to a predetermined control program on the basis of signals from various vehicle condition detecting means. The suspension properties of the suspension system can be changed by changing the control pattern of fluid supply to the fluid pressure cylinder devices so that, for instance, suspension properties in which importance is attached to the driving comfort or suspension properties in which importance is attached to the running stability can be obtained as desired.
As one of such active suspension systems, there has been known one in which a gas spring is provided for each fluid pressure cylinder device in communication therewith. With this arrangement, control of fluid supply to the fluid pressure cylinder devices is facilitated since the air springs absorb high-frequency vibrations such as road noise and accordingly control of fluid supply to the fluid pressure cylinder devices may solely depends upon displacements of the vehicle body at a low frequency (e.g., lower than 5 Hz) generated by operation of the driver such as rolling of the vehicle body.
In such an active suspension system, the conditions of the gas springs (the volume of the gas chamber, the pressure receiving area and the initial pressure) should be set on the basis of load acting on the wheels in state where the vehicle body is at rest horizontally (to be referred to as "1G state", hereinbelow), and the suspension stroke. However, since the volume of the gas chamber is limited by the space where the gas spring is disposed, the conditions of the gas springs are set solely on the basis of the volume of the gas chamber limited by the space where the gas spring is disposed and the stroke of wheel displacement when the vehicle moves to the normal running state from the 1G state. However, depending on the running condition, the fluid pressure in the fluid pressure cylinder devices can fall excessively, and the system is generally arranged so that it interrupts the control in order to prevent occurrence of damage when the fluid pressure in the fluid pressure cylinder devices becomes excessively low.
This arrangement gives rise to the following problem. That is, for example, when the pressure in the fluid pressure cylinder devices for the inner wheels reaches the threshold pressure (release limit pressure) while the vehicle is making a sharp turn, the control is interrupted at the moment. When the control is interrupted in the course of a turning, the vehicle body begins to roll toward the outer side of the turning (basically does not roll when it is under control) in the range of the suspension stroke and the fluid pressure in the fluid pressure cylinder devices for the inner wheels falls below the initial pressure of the gas springs communicating with the fluid pressure cylinder devices, whereby the grounding load of the inner wheels is nullified. When the inner wheel the grounding load of which is nullified is a driving wheel, the inner driving wheel races and the outer driving wheel is not driven by the action of the differential, whereby the driving force is removed in the course of the turning and the vehicle can spin. These problems can be avoided when the volume of the gas chamber and the pressure receiving area of the gas spring are sufficiently large (i.e., when the initial pressure is sufficiently low), which is very difficult to realize due to the limited space as described above.